Rise and Fall of the Himalaya
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214 T h e A l p i n e J o u r n A l 2 0 1 3 MIKE SEARLE Rise and Fall of The Himalaya Chokyi Dronma in a 17th century mural painting at Nyemo Chekar monastery (Hildegard Diemberger) Hindu, and the modern environmentalism that he had learnt by working with conservation biologists. These overlaid each other in the suggestive scenery. He then pulled out from his rucksack a brochure of a local envi- ronmental NGO he was part of: Kangchendzonga dominated the front cover; biodiversity was declared as a treasure of the ‘Great Snow Moun- tain’ enshrining the ‘Five Treasures’. The name of the holiest mountain st of Sikkim reflecting its ancient mythology had apparently acquired a 21 Fig. 1. The Himalaya and Tibetan Plateau looking west as viewed from the century dimension. Space Shuttle. Major geological features are shown. n 5 May 2012 a huge rockfall broke away from the summit ridge of References OAnnapurna IV, cascading debris down some 3000 metres into the Bauer, K (2006) ‘Common Property and Power: Insights from a Spatial upper reaches of the Seti valley. More than 70 people died in mudflows Analysis of Historical and Contemporary Pasture Boundaries among that swept down the river. When we reached the area one week later brown Pastoralists in Central Tibet’, Journal of Political Ecology 13, 24–47. muddy waters were still flowing down the Seti and into Pokhara itself. Diemberger, H (2007) When a Woman becomes a Religious Dynasty: the Compared to the massive catastrophic events in the geological record, the Samding Dorje Phagmo of Tibet. New York: Columbia University Press. Seti khola flood was only a minor event, yet it serves as a reminder of the Diemberger, H (1997) ‘Beyul Khenbalung, The Hidden Valley of Artem- on-going nature of the greatest continental collision in Earth history – the isia: On Himalayan Communities and Their Sacred Landscape’ in A rise and fall of the Himalaya. It is a story that I have been piecing together Macdonald (ed.) Mandala and Landscape. New Delhi: Printworld. over 30 years as a field geologist. This year (2013) I set out my unravelling Diemberger, K (1999) The Kurt Diemberger Omnibus. London: Bâton Wicks. of the geology of the Himalaya in Colliding Continents (see Reviews, p 338); Macdonald, A (1973) ‘The Lama and the General’ in Kailash, I, 225-233. what follows is but a snapshot of a complex and awe-inspiring drama. 215 216 T h e A l p i n e J o u r n A l 2 0 1 3 h i ma l A y A n g e o l o g y 217 Dhaulagiri S Partial melts Leucogranite 8167m N Bt Grt Ky Sil dykes Main Kalopani Central shear zone Nilgiri Fm. Thrust Ordovician V IV Annapurna Fm. II III cal Larjung unit Cambrian calc-silicate gneiss c - Unit I cal Kfs augen gneiss ~500 Ma silicat Dana quartzite c-s e - Gandrung quartzites Kyanite gneiss ilicate gneiss marble Annapurna Detachment Kuncha pelite pelite Ulleri auge s s Deurali Detachment LH Kuncha pelites ~1831 Ma Mylonite n gneiss s MCT s Beg Khola Tatopani Dana Ghasa Larjung Tukuche Fig. 2. Cross section of the Dhaulagiri Himal viewed from Poon Hill showing the major geological features. The main index minerals biotite (Bt), garnet (Grt), kyanite (Ky) and sillimanite (Sil) reveal the inverted metamorphism characteris- tic of the Himalaya. (Mike Searle) The Himalaya result from the collision of the Indian tectonic plate Fig. 3. South face of Annapurna I (8091m) composed of Cambrian-Ordovician with Asia some 50 million years ago, one of the largest continental colli- limestones and dolomites. (Mike Searle) sions the Earth has experienced in the last 500 million years (Fig. 1). 130 million years ago the Indian sub-continent was attached to Madagascar, south over colder rocks. Characteristic metamorphic index minerals range East Africa and Antarctica as part of the super-continent Gondwana. As from low-grade biotite and garnet along the base of the thrust slab in the this super-continent broke up India detached from its neighbours, pushed south through staurolite, kyanite and sillimanite grade, and eventually to apart by newly formed oceanic spreading centres beneath the floor of the migmatite (a partially molten rock) and granite as you progress upwards Indian Ocean, and began its rapid drift northwards. In just 50 million years and northwards. The granites are pale-coloured igneous rocks composed the Tethys ocean, that once lay between India and Asia, closed as India largely of quartz and feldspar but also containing red garnet, white shiny collided with Asia. Folding and thrusting of the rocks along the colliding muscovite mica and a characteristic black mineral, tourmaline. continental margins resulted in crustal shortening of hundreds of kilome- Mapping along the Himalaya has shown that along the base of this meta- tres and a doubling of the crustal thickness to 75 km. This crustal thickening morphic slab a huge shear zone or thrust fault carries all the metamorphic caused an increase of temperature and pressure and intense metamorphism rocks above, and the top of the slab is marked by a low-angle, normal fault of the rocks caught up in the collision. Shales became crystalline schists, termed the South Tibetan Detachment (Fig. 2). This feature cuts across limestones became marbles and, at the highest temperatures, the rocks even the summit region of Everest and clips the tops of many of the 8000 metre started to melt producing granites (which have been dated by uranium-lead peaks (for example Manaslu, Makalu and Kangchenjunga, as well as isotopic dating techniques to between about 24 and 19 million years old Shivling and the Bhagirathi peaks). Above the South Tibetan Detachment – Miocene). Subsequent uplift and erosion has exposed these granites in unmetamorphosed sedimentary rocks show spectacular folds and thrusts. many of the highest peaks along the Himalayan range. Annapurna and Dhaulagiri are two 8000m peaks composed of limestones In many older mountain ranges, such as the Highlands of Scotland and dolomites that form part of the unmetamorphosed Himalaya that lies for example, metamorphism is the right way up; in other words rocks above the South Tibetan Detachment (Figs. 3, 4). increase in metamorphic grade with greater structural depth, as one would expect. Along the Himalaya however, the entire metamorphic sequence Channel Flow Model is upside-down. The structures all dip to the north as India under-thrusts The structural profile across the Greater Himalaya reveals a mid-crust Tibet, but the rocks show increasing metamorphic grade the further north layer of rocks which were partially melted beneath southern Tibet before you trek toward higher structural levels. Hotter rocks have been thrust being extruded out to the south during the Miocene (approximately 24-15 218 T h e A l p i n e J o u r n A l 2 0 1 3 h i ma l A y A n g e o l o g y 219 S Everest N S N ZSZ Dhaulagiri 8172m PreCCambOrd sediments grt STD st ky Ordovician sill migmatite Nilgiri limestones GHS leucogranite S Greater Himalaya Tibetan Plateau N folded metamorphic GREATER HIMALAYAN SEQUENCE isograds Middle crust MAIN SOUTH CENTRAL TIBETAN THRUST DETACHMENT km 0 20 Ma INDIAN SHIELD ARCHAEAN Seismogenic upper crust GRANULITES 15 Ma Calc-mylonites 10 Ma Leucogranites Moho Cambrian limestones 50 50 Ma Partially molten middle crust MANTLE HighP granulite lower crust 100 GHS deep earthquakes (~6080 km) HighP Granulites (Dry) at base of crust Cambrian Annapurna Formation Cpx + Tr ± Kfs marbles N S 600ºC; 9 kbar Bt+Phl Greater Himalayan Sequence Dzakaa chu Tr+Kfs migmatite Kyanite gneiss + melts Annapurna 600ºC; 10-12 kbar; 35 Ma detachment sill Di+Kfs M.C.T. st ky ductile Lesser grt shear Fig. 4. Aerial view of the southeast face of Dhaulagiri (8167m) showing zone Himalaya Cambrian limestones above mylonites of the South Tibetan Detachment, brittle fault dipping gently to the north. Bt – biotite, Phl – phlogopite, Cpx – clinopyroxene, Tr – tremolite, Kfs – potassium feldspar, Di – diopside, GHS – Greater summit Himalayan Sequence. (Mike Searle) Rongbuk S Everest Tibet Plateau N km million years ago). These rocks are bounded by large-scale ductile shear 0 10 20 30 40 50 60 70 80 90 100 110 120 10 zones, the Main Central Thrust along the base and the low-angle normal folded Tethyan sediments 5 QD 5 km LD fault, the South Tibetan Detachment along the top. This model, dubbed 1 kb 0 the ‘Channel Flow model’ was formulated based on the structures and the 2 kb High Himalayan STD metamorphism, in particular accurate pressure and temperature determi- km 5 3 kb crystalline 4 kb rocks 3.7 10 4.1 nations from the rocks across the entire mountain range (Fig. 5). It seems 5 kb 4.5 15 6 kb therefore that many of the granites that form the highest peaks along the LEUCOGRANITE 7 kb 7 kb 20 isobar MELTING Himalaya were formed at depths of 18–30 kilometres beneath southern ZONE 25 Assuming 1) average surface elevation of 5 km Tibet from Indian plate rocks that were under-thrust to the north during 2) pressure gradient of 3.5 km / kbar 0.285 kbar / km the collision, metamorphosed, melted and then extruded back to the south 3) constant 10°N dip along STD during the Miocene. As the partially molten mid-crustal channel was Fig. 5. The Channel Flow model for the Himalaya during the Miocene showing extruding out to the south, the metamorphic isograds (lines of equal pres- the mid-crustal partially molten layer (in red) extruding south beneath the sure-temperature) were folded around this hot core, such that the whole South Tibetan Detachment normal fault and above the Main Central thrust with bottom limb was inverted and the top limb remained right way-up (Fig.